Leak detector

ABSTRACT

A leak detector operating according to the counterflow principle comprises a first high vacuum pump that operates in series with a primary pump. A second high vacuum pump is branched at a connecting point and an entry side thereof is connected to a mass spectrometer. A light test gas in the form of helium flows through the second high vacuum pump in a direction opposite to a transport direction. A conduit connecting an inlet to the entry side of the first high vacuum pump is devoid of restricted flow zones and valves in such a way that the pumping capacity of the leak detector is high for helium, thereby reducing the response time thereof. A valve for blocking the conduit leading via the first high vacuum pump is arranged between the exit side thereof and the entry side of the primary pump.

The invention relates to a leak detector operating according to thecounterflow principle and comprising a first high vacuum pump the entryside of which is connected to the inlet of the leak detector, a secondhigh vacuum pump the entry side of which is connected to a massspectrometer, a primary pump the entry side of which is connected to theexit sides of the two high vacuum pumps, and a bypass connecting theinlet of the leak detector to the primary pump and including a firstvalve.

A leak detector of this kind is described in EP 0 283 543 A1. It servesto detect leaks in a vacuum-tight apparatus. To this end, a light testgas is introduced into the apparatus to be tested and this apparatus isplaced in a vacuum-tight room from which the available gas is suckedoff. Alternatively, test gas can be sprayed from outside onto a testpiece the interior volume of which is connected to the inlet of the leakdetector. In the sucked-off gas, constituents of the test gas can bedetected by a mass spectrometer and be evaluated. If there are test gasportions, this suggests a leak in the apparatus. The leak detectoraccording to the counterflow principle referred to comprises a primarypump and a first high vacuum pump which are operated in series. A secondhigh vacuum pump connects the mass spectrometer to the entry side of theprimary pump. In a suction stage, the way via the first high vacuum pumpis blocked by closing a valve arranged upstream of this pump and thecontainer including the test piece is pumped dry via a bypass includinga valve. When the pressure in the container has decreased below a valueof about 100 mbar, a throttle valve in the conduit leading to the firsthigh vacuum pump is opened so that test gas may flow through the firsthigh vacuum pump. If, in the course of this, test gas (helium) entersinto the conduit system, it enters into the second high vacuum pump inthe counterflow and through the latter into the mass spectrometer. Ifhelium has not been detected by the mass spectrometer in this stage, afurther leak detection with higher sensitivity is started by releasingthe connection of the test piece to the first high vacuum pump throughanother valve of a large nominal diameter. The preevacuation valve inthe bypass is closed then. Thereafter, the test piece is evacuated to apressure of about 10⁻⁴ mbar, leakage rates in the order of about 10⁻⁵ to10⁻¹⁰ mbar l/s being detectable.

Because of the valves arranged in front of the entry side of the firsthigh vacuum pump, this pump is able to operate continuously at fullspeed under vacuum conditions. The valves arranged upstream, however,have a flow resistance and thus a throttling effect. Therefore, the fullsuction capacity of the high vacuum pump is not available at the testconnection in a non-throttled manner. Particularly, the helium suctioncapacity of the high vacuum pump cannot be utilized to the full extentthereof which would be convenient to achieve minimum response times.

It is the object of the invention to provide a leak detector accordingto the counterflow principle, which has an increased suction capacityfor helium at the inlet and thus short response times so that the searchfor leaks is shortened.

This object is solved, according to the invention, with the features ofclaim 1. Accordingly, the first high vacuum pump is connected to theinlet of the leak detector directly and without any valve. A secondvalve—apart from the first valve included in the bypass—is providedbetween the exit side of the first high vacuum pump and the primarypump.

In comparison with the described state of the art, the invention offersa number of advantages. There is no valve in front of the inlet of thefirst high vacuum pump. Consequently, this pump stands still at thebeginning of a pumping cycle and is either started simultaneously withthe opening of the first valve included in the bypass or a little bitlater when the pressure has already decreased. At the test connection,i.e., at the inlet of the leak detector, the full suction capacity ofthe first high vacuum pump is available in a non-throttled manner assoon as the pressure has fallen below the maximum intake pressure ofthis pump. Thus, the full helium suction capacity is particularlyavailable to achieve minimum response times. The detection sensitivityis not impaired since it is independent of the suction capacity and theinlet pressure of the first high vacuum pump. The helium leakage gasflow is always equivalent to the flow emerging from the leak.

The mentioned second valve has two possible operating modes. In thefirst operating mode, the first and the second valves are openedsimultaneously with the start of the first high vacuum pump. Thisresults in a maximum suction capacity of the primary pump through thefirst high vacuum pump and the bypass extending parallel thereto, whichresults in minimum pump-out time. In the second operating mode, thesecond valve remains closed for the moment until the pressure has leftthe viscous flow range which happens at approximately 0.1 to 1 mbar.Only then is the first high vacuum pump started and simultaneously, thesecond valve is opened. This operating mode offers an optimum protectionfrom dirt from the test piece since it is pumped through the bypass anddoes not enter the first high vacuum pump.

Preferably, the first high vacuum pump is arranged in horizontaloperating mode so that no particles are able to fall straight into itsentry side. If the primary pump is a pump sealed with oil, e.g., arotary valve pump, the test connection is optimally protected from oilreflux in the end pressure range.

Hereinafter, an embodiment of the invention is explained in detail withreference to the only FIGURE of the drawing.

A basic circuit diagram of the leak detector is illustrated in thedrawing.

The test piece 10 to be tested for leaks is filled with a test gas,typically helium, and inserted into a vacuum-tight test chamber 11. Inthe test chamber 11, a vacuum is generated by suction, and the gasleaving the test chamber is tested for helium portions.

To a connection 12 of the test chamber 11, the inlet 13 of the leakdetector 14 is connected. The entry side 15 of a high vacuum pump 16 isconnected with the inlet 13. Within the scope of the presentspecification and the claims, a high vacuum pump is a molecular pump. Amolecular pump exerts pulses upon gas molecules and accelerates them.Consequently, the molecular pump operates only at very low pressures,the outlet pressure on the pressure side amounting to only a few mbar. Atypical high vacuum pump is a turbomolecular pump comprising numerousstator discs and rotor discs, the rotor discs rotating at a high speed.A molecular pump generates a high compression for heavy molecules only,but a low compression for light molecules. A molecular pump has adifferent suction capacity for each gas. For the light gas helium, thesuction capacity is especially low.

The exit side 17 of the high vacuum pump 16 is connected with the entryside 19 of a primary pump 20 via a valve 18 yet to be explained. Theexit side 21 of the primary pump leads into the atmosphere. The primarypump 20 is a positive-displacement pump, e.g., which generates the lowpressure required for the operation of the high vacuum pump 16.

With its entry side 23, a second high vacuum pump 22 is connected with amass spectrometer 24 suitable for detecting the test gas helium. Via avalve 26, the exit side 25 of the second high vacuum pump 22 isconnected with the entry side of the primary pump 20.

The test gas is a light gas passing the high vacuum pump 22 incounterflow, i.e., in opposite direction to the direction of delivery,and reaches the mass spectrometer 24.

Further, the leak detector comprises a bypass 30 connecting the inlet 13with the entry side 19 of the primary pump 20 and including a firstvalve 31 so that it can be optionally opened and blocked.

A control 32 controls the first valve 31 and the second valve 18connected with the exit side 17 of the high vacuum pump 16 in dependenceon the pressure at the inlet 13 which is measured by a pressure gauge33. Further, the inlet 13 is connected to the atmosphere via aventilation valve 34. Likewise, the entry side 19 of the primary pump 20is connected to the atmosphere via a ventilation valve 35.

The pressure at the exit side 25 of the second high vacuum pump 22 ismeasured by a pressure gauge 36. Another pressure gauge 37 is connectedwith the exit side 17 of the first high vacuum pump 16. In dependence onthe measured pressure, the valve 26 is opened when the pressure fallsbelow a predetermined value.

The substantial feature of the invention consists in that the conduit 40connecting the inlet 13 with the entry side 15 of the first high vacuumpump 16 is completely non-throttled and neither includes a restrictedflow zone nor a valve. Thus, the complete suction capacity of the highvacuum pump 16 is available at the inlet 13 as soon as the pressurefalls below the maximum suction pressure of the high vacuum pump.Thereby, the shortest response times for the test gas helium areachieved.

The conduit branch including the first high vacuum pump 16 is blocked bythe second valve 18.

In the first operating mode, the valves 31 and 18 are openedsimultaneously with the start of the high vacuum pump 16. Thereby, aminimum pump-out time is achieved by a maximum suction capacity of theprimary pump 20, both through the high vacuum pump 16 and through thebypass 30.

In the second operating mode, the valve 18 first remains closed untilthe pressure at the inlet 13 has fallen below a limit value of about 0.1to 1 mbar, which represents the limit of the viscous flow range. Onlywhen the signal of the pressure gauge 33 indicates that the pressure hasfallen below the limit value, the high vacuum pump 16 is started, andsimultaneously, the second valve 18 is opened. In this operating mode,the high vacuum pump 16 is protected from dirt from the test pieceduring the pumping out of the gas since the gas is exclusively ledthrough the bypass 30.

The second high vacuum pump 22 comprises two intermediate inlets 41 and42 each of which is connected to the exit side 17 of the first highvacuum pump 16 via switchable valves 43 and 44, respectively. Byswitching these valves, the measuring range can be changed. Hereinafter.examples for pressures at characteristic sites of the high vacuum pump22 are set forth:

-   exit side 25: 15 mbar-   intermediate inlet 42: 1 mbar-   intermediate inlet 41: 10⁻² mbar-   entry side 23: 10⁻⁴ mbar.

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 5. Leak detectoraccording to the counterflow principle, comprising: a first high vacuumpump having an entry side that is connected to an inlet of the leakdetector; a second high vacuum pump having an entry side which isconnected to a mass spectrometer; a primary pump having an entry sidewhich is connected to exit sides of the first and second high vacuumpumps; and a bypass connecting the inlet of the leak detector to theprimary pump and including a first valve, wherein the first high vacuumpump is connected to the inlet of the leak detector in a non-throttledmanner and without valving, wherein a second valve is provided betweenthe exit side of the first high vacuum pump and the primary pump. 6.Leak detector according to claim 5, wherein the first high vacuum pumpis started simultaneously with the opening of the second valve uponopening the first valve.
 7. Leak detector according to claim 5, whereinthe first high vacuum pump is activated only after the first valve hasbeen opened when the pressure at the inlet has left the viscous flowrange or fallen below a limit value.
 8. Leak detector according to claim5, wherein the second high vacuum pump further comprises at least oneintermediate inlet connected to the exit side of the first high vacuumpump via a valve, said valve being dependently controlled based upon thepressure of the exit side of the first high vacuum pump.